1. Field of the Invention
The present invention relates to a motor operated power steering device which incorporates a torque limiter to combat the application of excessive torque to the steering shaft.
2. Related Art
In a motor operated power steering device which incorporates a steering shaft, a gear fitted around the circumference of the steering shaft, and a motor that transmits its rotational force to the steering shaft by way of the gear for the purpose of assisting the steering operation, the motor is seriously damaged if excessive torque is applied to the steering shaft as a result, for example, of the driver's vehicle mounting a roadside curb. Again, if output is increased by using the gear to increase the deceleration ratio of the rotational speed of the motor, the diameter of the gear must also be increased in order to combat the impact load arising out of the torque action on the steering shaft. This serves to prevent the device from being made more compact. For the reasons outlined above, a torque limiter is provided between the motor and the steering shaft.
In the past, the torque limiter are normally incorporated into the motor. These mechanisms are operated by causing the motor's output shaft to slip whenever the torque acting on the shaft has reached a predetermined limit value.
However, when the torque limiter is incorporated into the motor in this way, it is prevented from functioning if the motor's output shaft is locked for some reason such as the occurrence of an accident. In this sort of situation, the steering wheel becomes unusable. Another problem is that the teeth of the gear which transmits the rotational force of the motor to the steering shaft are sometimes made of a synthetic plastic in order to reduce noise. Teeth of this sort is broken by the application of excessive torque and this causes the gear rotation to lock. The torque limiter incorporated into the motor does not function under circumstances of this sort and the steering wheel becomes unusable. Another problem is that in transmission systems in which the rotational force of the motor is transmitted to the steering shaft by way of gears, the efficiency of transmission of the gears tends to vary by the individual difference of the gears. It is for this reason difficult to set the limit torque value precisely within the predetermined setting range. Again, the ratio of transmission of the rotational force through the gears, such as a worm wheel and a worm is increased. In this sort of case, the limit torque value must be set very precisely and this means using a high precision torque limiter.
One possible alternative to the above configurations is to fit a torque limiter in between the gear and the steering shaft. For example, the motor operated power steering device disclosed in Japanese patent laying open gazette H2-120178 has a mechanism which generates a force in order to pinch a plurality of friction plates, that rotate in unison with the gear, and a plurality of friction plates, that rotate in unison with the steering shaft, in the axial direction of the steering shaft. The limit torque is in this case set in accordance with the frictional resistance between the different sets of friction plates. This device is also provided with a mechanism which enables the limit torque value to be adjusted by varying the force with which the friction plates are pinched.
The above mechanism which needs a plurality of friction plates to be pinched, however, call for the use of a large number of parts while at the same time increasing the number of man/machine hours required for manufacture of the device. The limit torque value is also altered-by the cumulative effect of inaccuracies in the axial dimensions of the friction plates. It is difficult under these circumstances to ensure that the limit torque value is set accurately within the predetermined setting range. Again, the need for the limit torque value adjustment mechanism also increases the number of parts required and complicates the configuration of the device.
In the motor operated power steering device disclosed in Japanese utility model laying open Gazette publication No. H2-15576, the limit torque is set in accordance only with the press fit force with which the steering shaft is forced into the gear.
However, if the limit torque value is set in accordance only with the press fit force with which the steering shaft is forced into the gear, the press fit force must be gauged very precisely. This means that the outer circumference of the steering shaft and the inner circumference of the gear must be machined extremely accurately. This degree of machining accuracy is in practice difficult to achieve and this makes it difficult to ensure that the limit torque is set precisely within the predetermined setting range.
In an alternative motor operated power steering device disclosed in the Japanese utility model laying open gazette No. H2-15576, the gear is fitted such that it is able to move in an axial direction along the steering shaft. The gear is pressed onto a member fitted on the steering shaft via a plurality of friction plates by an axial force derived from the axial deformation of a spring. In this case the limit torque value is set in accordance with the axial force. This device is also fitted with a mechanism which enables the limit torque value to be adjusted by varying the axial deformation of the spring.
However, for the mechanism which presses the gear onto the member fitted on the steering shaft via the plural friction plates by the axial force derived from the axial deformation of the spring, a large number of parts are required and a greater number of man/machine hours is required to complete the manufacturing process. The limit torque is also altered by the cumulative effect of inaccuracies in the axial dimensions of the friction plates and the gear. It is difficult under these circumstances to ensure that the limit torque value is set accurately within the predetermined setting range. Again, the need for the limit torque adjustment mechanism also increases the number of parts required and complicates the configuration of the device. Also, if the gear is a worm wheel or the like, when a rotational force is transmitted to the clockwise direction by the gear, an axial force acts in one direction on the gear, and when a rotational force is transmitted to the counterclockwise direction by the gear, an axial force acts in the other direction on the gear. Thus, when the rotational force is transmitted to the clockwise direction, the gear moves by deforming the spring in an axial direction. However, when the rotational force is transmitted to the counterclockwise direction, the gear comes into contact with the member fitted on the steering shaft, so that the gear can not move. Therefore, the limit torque value at the time when the steering shaft is rotating to the clockwise direction differs from the limit torque value at the time when it is rotating to the counterclockwise direction.
It is the object of the present invention to provide a motor operated power steering device which can resolve the problems outlined above.